CYP3A4 inhibitors and inducers list: strong, moderate and weak, and the substrates that matter

The classification is quantitative rather than descriptive. A strong inhibitor raises the plasma exposure of a sensitive CYP3A4 substrate at least fivefold, or reduces its clearance by at least 80 percent. A moderate inhibitor produces a two to fivefold rise, and a weak inhibitor a 1.25 to twofold rise. These thresholds come from the FDA drug interaction guidance and are what product labeling refers to when it says a dose adjustment is required with strong inhibitors.

The practical value of the categories is that labeling is written in the same language. When an oncology drug label says to reduce the dose with a strong CYP3A4 inhibitor and to avoid strong inducers, you do not need a drug-by-drug interaction table; you need to know which category the other drug falls into. That single mapping resolves a large share of interaction questions.

A few combinations account for a disproportionate share of reported serious events. Simvastatin or lovastatin with a strong inhibitor such as clarithromycin or itraconazole raises the risk of rhabdomyolysis, and it is common because both prescriptions are ordinary. Colchicine with clarithromycin has caused fatal toxicity, particularly in renal impairment where both clearance routes are compromised. Ergot alkaloids with strong inhibitors can cause severe vasospasm.

On the induction side the harm is quieter and often only recognized in retrospect. Rifampin started for tuberculosis or a prosthetic joint infection can drop concentrations of an immunosuppressant, a direct oral anticoagulant, an antiretroviral or a hormonal contraceptive below the effective range. Nothing hurts on day one. The graft rejects, the clot forms, or the pregnancy happens weeks later, and the connection back to the prescription is easy to miss.

Yes, and it is a genuine strong inhibitor at meaningful quantities rather than a curiosity. Furanocoumarins in grapefruit irreversibly inactivate intestinal CYP3A4, so the effect is concentrated on first pass metabolism of orally administered substrates and persists until the enzyme is regenerated, which takes on the order of a day or more. That is why separating the dose from the juice by a few hours does not solve it.

The substrates where it matters most are the ones with high first pass metabolism and a narrow margin: simvastatin, lovastatin, some calcium channel blockers, tacrolimus, and a number of oncology agents. Seville oranges and pomelo contain similar compounds. Most other citrus does not. It is worth asking about specifically, since patients do not consider juice a medication and will not mention it.

A great many CYP3A4 substrates are also substrates of P-glycoprotein, the efflux transporter in the gut wall and blood brain barrier, and many inhibitors hit both. Ritonavir, clarithromycin, itraconazole, verapamil and amiodarone are all dual inhibitors. When both pathways are blocked at once, exposure rises more than either mechanism alone would predict, which is why apixaban and rivaroxaban labeling singles out combined CYP3A4 and P-glycoprotein inhibitors rather than CYP3A4 inhibitors generally.

The same overlap operates in the induction direction. Rifampin induces both, which is why it reduces direct oral anticoagulant exposure so effectively. Keeping the two mechanisms in mind explains several interactions that look anomalous if you think about CYP3A4 alone.

Knowing that clarithromycin is a strong CYP3A4 inhibitor is the easy part. Noticing it at the moment you prescribe it to a patient whose home list includes simvastatin, apixaban and tacrolimus, at the end of a full clinic, is the hard part. That is a memory and attention problem, not a knowledge problem, and it is what decision-support is actually for.

Prescriber.io checks the regimen you enter for interactions including CYP3A4 inhibition and induction, states the mechanism in plain language with the source cited, and returns it alongside contraindications, allergy blockers, renal and hepatic dose considerations and guideline-based alternatives, in one card. It is decision-support for licensed US clinicians, never autonomous prescribing. You review the flag, verify against the current labeling, and sign.